The present invention relates to a novel phosphazene compound and to a lubricant containing the phosphazene compound. More particularly the invention concerns with a compound to be used in a lubricant for a recording medium such as hard disks of large-capacity recording medium, and metal vapor-deposited tapes for 8 mm video tape recorder or digital video casette, and with a lubricant containing the compound.
Thanks to prevalent internet, intranet, local area network, etc, information can be fetched easily only by gaining access to a network. One of those which support this multimedia era is external recording devices. Of existing large-capacity external recording devices, those of the so-called CSS method are usually employed in which a recording medium layer is provided on a disk substrate having a high strength (hard disk), and the hard disk is rotated at high speed to operate a recording/reproducing element (head). Examples of the hard disk are a stationary magnetic disk, optical disk and optical magnetic disk. Of these, the most prevalent is a device employing the stationary magnetic disk. In order to increase the capacity of these external recording devices without changing their size, it is necessary to increase the surface recording density of a recording medium layer, for example. This requires that bit size be reduced and the head be brought near the recording medium as much as possible. To reduce a clearance between the head and disk, the unevenness of the disk surface should be minimized. For instance, to obtain a clearance of not more than 100 nm, the unevenness should be not more than 10 nm. However, if the smoothness of the disk surface is increased, the head is liable to be adsorbed onto the disk surface. To obtain high-speed response, it is necessary to increase bit transfer rate and therefore the rotational speed of the disk. For instance, a 3.5-inch hard disk requires a maximum number of revolution of 5400 to 7200 rpm, and the speed of the head as it passes through the periphery of the disk reaches 125 km/hr. Any of the above-mentioned modifications, however, increases considerably the probability of contact between the disk and head, as well as friction therebetween, thus decreasing the reliability.
In view of the foregoing, for protection of the disk surface and head, it is the simplest way to increase the amount of a lubricant applied to the surface of a protective coat for a disk. However, if the amount of a lubricant applied is increased, not only the slide resistance of the lubricant layer is increased but also the adsorption of the lubricant is enhanced, thereby causing a stop or burning of a spindle motor. For decreasing slide resistance, if used a lubricant of low viscosity, the lubricant moves and disappears from a protective layer during high-speed rotation of the disk. This causes a contact between the disk and head, and their breakage.
A lubricant, in an extremely simple manner, forms a lubricant film having a uniform thickness of molecules on a protective layer provided on a hard disk such as to reduce friction and wear which may be caused during the rotation of the hard disk and at the start and stop of the device. For protection of the disk and head, there is required a lubricant which has a low slide resistance, large bonding strength to a protective layer, low coefficient of friction, high chemical stability, high thermal stability, high corrosive resistance and lubricity at low temperatures of below xe2x88x9210xc2x0 C., has a low vapor pressure, and which can form ultrathin films. A lubricant for hard disk is required to have no change in coefficient of friction over 50,000 times in CSS durability. That is, the lubricant should be free from abnormalities such as seizing of the head and blemish of the disk up to at least 50,000 times in a CSS repeated test.
From the point of view of chemical stability, strength, ultrathin film formation and smoothness, Diamond Like Carbon (DLC) is used in the surface protective layer of existing hard disks. This protective layer is, as apparent from its structure, mainly composed of carbon, which leads to a low chemical bonding. Its adhesion therefore merely depends upon hydrogen bond with hydrogen atom or nitrogen atom, and interaction such as van der Waals force.
Thus, perfluoroalkyl polyether oligomer (hereinafter referred to as xe2x80x9cPFPExe2x80x9d) is usually employed as a lubricant (Monthly magazine xe2x80x9cTribologyxe2x80x9d, No. 99, November, pp37-38, 1995). PFPE has a low surface energy, chemical inertness, low vapor pressure, and thermal stability, which characteristics are derived from fluorocarbon. Unfortunately, it has a drawback that the bonding strength to the protective layer surface is small because of low surface energy, and the lubricant moves and disappears from the protective layer by centrifugal force during the rotation of a disk. To suppress this, an additive of a secondary component is used (U.S. Pat. No. 4,871,625), however, low compatibility deriving from the fluorocarbon of PFPE causes a phase separation, failing to obtain sufficient effect. In order to afford affinity in PFPE, there has been proposed one method in which a functional group, such as hydroxyl group or acylamide group, is introduced into the terminal or center of the main chain of PFPE (FOMBLIN Z Derivative, Ausimont S.p.A). Unfortunately, the functional group having such an active hydrogen might decrease chemical stability due to the friction with the head.
Perfluoroalkoxy phosphazene compounds are disclosed for treatment of magnetic recording medium (JP-A-63-103428). When these compounds are used as a lubricant for hard disks, the hard disks can not achieve sufficient CSS durability. Thus, there is a demand for a novel lubricant.
An object of the invention is to provide a compound to be used in a lubricant, e.g., for hard disks which have excellent CSS durability, and a lubricant containing such compound.
The present invention relates to a phosphazene compound of the formula (1) shown below which has an arrangement such that perfluoroxyalkylene units selected from xe2x80x94CF2Oxe2x80x94 and xe2x80x94CF2CF2Oxe2x80x94 are randomly distributed, the compound having a functional terminal group A
Axe2x80x94OCH2CF2O(CF2CF2O)p(CF2O)qCF2CH2Oxe2x80x94Axe2x80x83xe2x80x83(1) 
wherein A is a group of the formula (a) shown below, R is a hydrogen atom, C1-4 alkyl, C1-4 alkoxy, C1-4 haloalkyl, aryl, substituted aryl, aryloxy or substituted aryloxy, and p and q are a real number of 1 to 30. 
The present invention also relates to a phosphazene compound of the formula (5) which has an arrangement such that perfluoroxyalkylene units selected from xe2x80x94CF2Oxe2x80x94 and xe2x80x94CF2CF2Oxe2x80x94 are randomly distributed, the compound having a functional terminal group G
Gxe2x80x94[OCH2CF2O(CF2CF2O)p(CF2O)qCF2CH2OH]6-zxe2x80x83xe2x80x83(5) 
wherein G is a group of the formula (g) shown below, R is a hydrogen atom, C1-4 alkyl, C1-4 alkoxy, C1-4 haloalkyl, aryl, substituted aryl, aryloxy or substituted aryloxy, and p and q are a real number of 1 to 30, and z is an integer of 1 to 5. 
The invention also concerns with a lubricant containing at least one of the phosphazene compounds of the formulas (1) and (5).